The present invention relates generally to feature measurement in scanning electron microscopy, and more specifically to apparatus and methods for optimizing image quality. The present invention may also be applied to feature measurement and image enhancement in similar instruments.
FIG. 1 is a diagrammatic representation of a conventional scanning electron microscopy configuration 100. As shown, a beam of electrons 102 is scanned over a sample 104 (e.g., a semiconductor wafer). Multiple raster scans 112 are typically performed over a small area 114 of the sample 104. The beam of electrons 102 either interact with the sample and cause an emission of secondary electrons 106 or bounce off the sample as backscattered electrons 106. The secondary electrons and/or backscattered electrons 106 are then detected by a detector 108 that is coupled with a computer system 110. The computer system 110 generates an image that is stored and/or displayed on the computer system 110.
Although conventional microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the sample for some applications. For example, on a sample made of a substantially insulative material (e.g., silicon dioxide), performing one or more scans over a small area causes the sample to accumulate excess positive or negative charge in the small area relative to the rest of the sample. The excess charge generates a potential barrier for some of the secondary electrons, and this potential barrier inhibits some of the secondary electrons from reaching the detector 108. Since this excess charge is likely to cause a significantly smaller amount of secondary electrons to reach the detector, an image of the small area is likely to appear dark, thus obscuring image features within that small area. Alternatively, excess negative charge build up on the sample can increase the collection of secondary electrons causing the image to saturate.
Conventionally, various operating parameters of the microscopy system are manually adjusted until a clear image is obtained. For example, the sample stage voltage or beam source voltage are adjusted to obtain different landing energies that will result in a clearer image. The image quality is typically assessed by the microscopy operator, and the operating parameters are adjusted manually until the operator determines that the image quality is adequate. Since the individual judges image quality and the operating parameters are manually adjusted, this technique tends to be relatively subjective and time-consuming.
Thus, microscopy apparatus and techniques for improving image quality are needed. More specifically, mechanisms for reliably and efficiently controlling charge distribution on the surface of the sample are needed.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for controlling surface charge on a sample by obtaining surface voltage values of the sample during a charged particle beam metrology or inspection procedure (e.g., in-situ). In general terms, a surface charge value (e.g., by measuring surface voltage) is obtained in-situ under a first set of operating conditions (e.g., a predefined beam landing energy). It may then determined whether the surface charge is at a predetermined optimum value (e.g., zero charge build-up). The operating conditions may then be adjusted until the surface charge reaches the predetermined optimum value or optimum conditions may be extrapolated from previously attempted operating conditions. The charged particle beam metrology or inspection procedure may then be performed under the optimum operating conditions.
In one embodiment, an apparatus for generating an image from a sample is disclosed. The apparatus includes a charged particle beam generator arranged to generate and control a charged particle beam substantially towards a portion of the sample and a detector arranged to detect charged particles originating from the sample portion to allow generation of an image from the detected charged particles. The apparatus further includes a measurement device arranged to measure a characteristic of the sample portion to obtain a surface voltage value of the sample portion that is exposed to the charged particle beam. In a preferred embodiment, the measurement device is an electrostatic voltmeter positioned to obtain a surface voltage value of the exposed sample portion.
In another aspect, the invention is directed towards a method for controlling charge. A charged particle beam is directed substantially towards a portion of the sample under a first set of operating conditions. A surface charge value of the sample portion is obtained under the first set of operating conditions. It is then determined whether an optimum set of operating conditions associated with a predetermined surface charge value have been found. When the optimum conditions have not been found, the operating conditions are adjusted and the charged particle beam is directed substantially towards the sample portion. When the optimum conditions have been found, the charged particle beam is directed substantially towards the sample portion under the found optimum operating conditions. In another embodiment, the present invention pertains to a computer readable medium having computer code for performing these tasks.
In yet another embodiment, a charged particle beam measurement device for obtaining an image of a portion of a sample is disclosed. The charged particle beam measurement device includes a source unit arranged to generate and direct an incident charged particle beam substantially towards a portion of the sample, a first detector arranged to detect charged particles emitted from the sample portion, and an image generator arranged to generate an image from the detected charged particles. The charged particle beam measurement device also includes a second detector arranged to measure a characteristic of the sample portion that is related to a surface voltage value of the sample portion after or while the incident beam hits the sample portion.
In another embodiment, the charged particle beam measurement device includes a source unit arranged to generate and direct an incident charged particle beam substantially towards a portion of the sample, a detector arranged to detect charged particles emitted from the sample portion, and an image generator arranged to generate an image from the detected charged particles. The measurement device also includes a grid positioned between the detector and sample, and the grid is coupled to a voltage source configurable to inhibit charged particles emitted from the sample having an energy less than the energy of the grid.
A method of controlling charge build up on a test sample that is to undergo a metrology or inspection procedure within a charged particle device is disclosed. The method includes(a) in a charged particle device, adjusting a focus setting of a charged particle device to obtain a first optimum image of a reference sample having a known surface charge value, wherein the first optimum image corresponds to an optimum focus setting; (b) selecting a first set of operating conditions for the charged particle device; (c) in the charged particle device, directing a charged particle beam towards a test sample having an unknown surface charge value; (d) in the charged particle device, adjusting the focus setting of the charged particle device to obtain a second optimum image of the test sample, wherein the second optimum image corresponds to a current focus setting; and (e) when the current focus setting equals the optimum focus setting, determining that the unknown surface charge equals the known surface charge and commencing with the metrology or inspection procedure under the first set of operating conditions.
The present invention has several associated advantages. For example, since surface voltage is measured in-situ, surface charge may be accurately and objectively determined to optimize the operating conditions. This is in stark contrast to conventional methods that where the user subjectively determines the quality of the image to adjust operating conditions accordingly. Additionally, optimum conditions are quickly and reliably determined with by automatically obtaining surface voltage values.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.